1. Field of the Invention
This invention pertains to the field of signal processing for video cameras and, in particular, to an automatic color balance circuit for a video camera.
2. Description Relative to the Prior Art
A video camera is correctly color-balanced when it reproduces a picture of a white card as a neutral white without any identifiable hue. (As a result, color balance for a video camera is often referred to as white balance.) Such a balance is ordinarily obtained when the card is illuminated with the light source used for recording and the red, green, and blue signal channels provide equal output amplitudes. If the light source is changed, such as in going from an indoor (tungsten or fluorescent lighting) to an outdoor setting, the camera must be rebalanced to provide the same output in relation to the white card with the new source of lighting (in this case, the sun). The usual practice is to hold the gain of the green channel fixed and alter the gain of the red and blue channels until the red and blue amplitudes match the green amplitude.
Many cameras now are automatic in that the red and blue amplitudes are matched to the green amplitude by an automatic gain control circuit at the touch of an "auto white" button. The procedure is to point the camera at a white card in the light used for recording, to frame the card so that it occupies the entire picture, and to press the "auto white" button. White balance is thereby effected although "auto" is somewhat of a misnomer since the camera user must manually engage the white balance circuit for each new lighting condition. Many cameras are also equipped with a memory circuit that stores the gain factors of the red and blue channels so that the camera will remain balanced for a particular light source.
It is clearly inconvenient to have to manually rebalance the red and blue channels whenever illumination conditions change, and especially inconvenient to do so in connection with a white card. Moreover, the illumination should come from as wide an area as possible so that color measurements relate to the color characteristics of incident light on the scene, rather than to reflected light from the subject, which may possess an unusual color bias. The tendency, consequently, has been to find ways of predicting white balance by comparing illumination levels for isolated areas of the ambient spectrum (that is, without resort to a white card). In U.S. Pat. No. 4,395,730 the ratios of red light to green light and blue light to green light from external sensors are used to effect an automatic white balance adjustment. In U.S. Pat. No. 4,584,598 the luminous intensities of two areas of the ambient spectrum (mercury and certain infra-red wavelengths) are used to identify the character of the light source.
An automatic white balance system that measures different regions of the ambient scene spectrum must content with a wide variation in illumination level, i.e., a wide dynamic range. This complicates the light measurement procedure because extreme light levels tend to drive the measurement toward its extremities, e.g., the extremities of a time-based measurement or of an analog-to-digital conversion. This is especially critical if white balance is based upon a ratio of illuminants, since poor resolution in the numerator and denominator leads, in many cases, to either meaningless values or ratio values indistinquishable one from the other. The U.S. Pat. No. 4,584,598 tries to deal with this problem by logarithmically compressing the light measurements prior to A/D conversion. Logarithmic compression at this point in the circuit, however, tends to reduce the resolution of light measurements, especially for extreme light levels.
Adopting a different approach, U.S. Pat. No. 4,646,161 discloses an automatic white balance control circuit which standardizes the red and blue light measurements with respect to the green light. All three colors are simultaneously integrated but only the green integration is directly controlled in relation to a reference level. When the green integration signal matches the reference level, all three integrations are sampled and held. In this way, the green signal is made constant regardless of the amount of incident light and the concurrent integrations of red and blue are standardized to a fixed reference--the green signal. This is said to produce increased stability in color balance. Nonetheless, a reference level has to be established with respect to some expected light condition. The dynamic range of ambient illumination is such that some measurements will be unsatisfactory as the green integration either quickly terminates in extremely bright light or drags on in extremely dim light.